Precision strip heating element

ABSTRACT

A heating element includes a continuous planar strip and a plurality of mounting members. A path of the continuous strip from a first end to a second end is circuitous and includes a plurality of repeating cycles, each of which includes a plurality of first straight segments, a plurality of second straight segments and a plurality of radiused segments. A length of the first straight segment is greater than a length of the second straight segment and an angular sum of a single cycle of the circuitous path is greater than 360 degrees. The heating element can be incorporated into a heating assembly for, as an example, semiconductor processing equipment.

RELATED APPLICATION DATA

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Ser. No. 61/202,206, filed Feb. 5, 2009, theentire contents of which are incorporated herein by reference.

FIELD

The present disclosure relates to heating elements. More particularly,the present disclosure relates to strip heating elements for furnaces,e.g., semiconductor processing furnaces, that have a circuitous pathincluding straight and radiused segments that advantageouslyaccommodates thermal expansion and contraction.

BACKGROUND

In the discussion of the background that follows, reference is made tocertain structures and/or methods. However, the following referencesshould not be construed as an admission that these structures and/ormethods constitute prior art. Applicant expressly reserves the right todemonstrate that such structures and/or methods do not qualify as priorart.

Conventional heating elements are generally formed of wire or sheetmetal of various designs and geometries. However, wire patternedelements are generally limited in operating temperature by virtue ofbeing embedded or semi-embedded in a surrounding medium, such asinsulation. Further, wire patterned elements are typically not precisionformed, are highly labor intensive and have a medium ratio of surface tomass resulting in fast heating and cooling. For sheet metal heatingelements, those formed as primarily square patterns suffer fromnon-uniformity, while those with continuously curving patterns producehigh stresses, both effects being more pronounced when the heatingelement expands at operating temperatures.

SUMMARY

A substantially uniformly radiating and substantially stress freeheating element, even at operating temperatures, would be advantageous.Such a heating element can be included in a furnace to improveprocessing of items. For example such a heating element can be includedin a semiconductor processing furnace for the processing ofsemiconductor wafers.

An exemplary heating element comprises a continuous planar strip,wherein a path of the continuous strip from a first end to a second endis circuitous and includes a plurality of repeating cycles, eachrepeating cycle including a plurality of first straight segments, aplurality of second straight segments and a plurality of radiusedsegments, wherein a length of the first straight segment is greater thana length of the second straight segment, and wherein an angular sum of asingle cycle of the circuitous path is greater than 360 degrees.

An exemplary embodiment of a heating assembly comprises the heatingelement mounted in spaced relation to the insulating substrate by aplurality of mounting members.

An exemplary method of manufacturing a heating assembly comprisesforming a heating element body from a resistance alloy, the heatingelement body including a continuous planar strip with an emittingsurface and a plurality of mounting members, bending the plurality ofmounting members out of plane relative to the continuous strip, andinserting the plurality of mounting members into a substrate until anintegrated spacer on the mounting members contacts the substrate,wherein a path of the continuous strip from a first end to a second endis circuitous and includes a plurality of repeating cycles, eachrepeating cycle including a plurality of non-parallel first straightsegments, a plurality of second straight segments and a plurality ofradiused segments, wherein a length of the first straight segment isgreater than a length of the second straight segment, and wherein anangular sum of a single cycle of the circuitous path is greater than 360degrees.

Another exemplary method of manufacturing a heating assembly comprisesforming a heating element body from a resistance alloy, the heatingelement body including a continuous planar strip with an emittingsurface, and inserting a plurality of mounting members through anopening integrally formed on the continuous strip and into a substrateuntil a spacer associated with the mounting members contacts thesubstrate, wherein a path of the continuous strip from a first end to asecond end is circuitous and includes a plurality of repeating cycles,each repeating cycle including a plurality of non-parallel firststraight segments, a plurality of second straight segments and aplurality of radiused segments, wherein a length of the first straightsegment is greater than a length of the second straight segment, andwherein an angular sum of a single cycle of the circuitous path isgreater than 360 degrees.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWING

The following detailed description can be read in connection with theaccompanying drawings in which like numerals designate like elements andin which:

FIG. 1 is a plan, schematic view of an exemplary embodiment of a heatingelement.

FIG. 2 is a plan, schematic view of a further exemplary embodiment of aheating element.

FIGS. 3A and 3B show two different perspective, disassembled views of anexemplary embodiment of a heating assembly.

FIGS. 4A and 4B show two different perspective, disassembled views ofanother exemplary embodiment of a heating assembly.

FIGS. 5A to 5C show portions of an exemplary embodiment of a heatingelement, including a first embodiment of a mounting member.

FIG. 6 shows portions of another exemplary embodiment of a heatingelement, including a second embodiment of a mounting member.

FIGS. 7A-D show, in plan view, several embodiments of a heating elementwith a combination of types of integrated mounting members and mountingmembers that are separate elements.

FIG. 8 shows a perspective, disassembled view of an exemplary embodimentof a heating assembly with a combination of types of integrated mountingmembers and mounting members that are separate elements.

FIGS. 9A and 9B illustrates the modeled temperature distribution for theexemplary embodiment of a heating element as generally shown in FIG. 1with a first embodiment of a mounting member (FIG. 9A) and with a secondembodiment of a mounting member (FIG. 9B).

FIG. 10 illustrates the modeled temperature distribution for theexemplary embodiments of a heating element as generally shown in FIG. 2.

FIG. 11 illustrates the modeled temperature distribution for a prior artheating element.

FIGS. 12A and 12B show two different perspective views of an exemplaryembodiment of a heating assembly with a plurality of heating elements.

FIGS. 13A-E show examples heating element installations.

DETAILED DESCRIPTION

An exemplary embodiment of a heating element 10 comprises a continuousplanar strip 12 and a plurality of mounting members 14. A path of thecontinuous strip 12 from a first end 16 to a second end 18 is circuitousand includes a plurality of repeating cycles 20. Each repeating cycle 20includes a plurality of non-parallel first straight segments 22, aplurality of second straight segments 24 and a plurality of radiusedsegments 26. An angular sum of a single cycle of the circuitous path isgreater than 360 degrees.

The heating element 10 has an emitting surface 30 that generally extendsin and generally is contained in a first plane. Within this first plane,the plurality of first straight segments 22 are oriented generallylaterally to an axis 32 oriented from the first end 34 of the heatingelement 10 to a second end 36 of the heating element 10, e.g., within±15 degrees of perpendicular to the axis 32. The plurality of secondstraight segments 24 are oriented generally longitudinally to the axis32, e.g., within ±15 degrees of parallel to the axis 32. In an exemplaryembodiment, any two consecutive first straight segments are generally(within ±15 degrees, alternatively within ±5 degrees) non-parallel andany two consecutive second straight segments are generally (within ±15degrees, alternatively within ±5 degrees) parallel. Alternatively, anytwo consecutive first straight segments 22 are strictly non-paralleland/or any two consecutive any two consecutive second straight segments24 are strictly parallel. The axis 32 is conventionally oriented in theX-axis direction.

In an exemplary embodiment, a single cycle 20 of the circuitous pathincludes two first straight segments 22, two second straight segments 24and four radiused segments 26. The single cycle 20 includes two lobes38. Each lobe 38 includes two radiused segments 26 and one secondstraight segment 24. The one second straight segment 24 separates thetwo radiused segments 26.

The radiused segment can take any suitable form. FIG. 1 is a plan,schematic view of an exemplary embodiment of a heating element 10. Thecycles 20 in the exemplary embodiment shown in FIG. 1 have radiusedsegments 26 that are continuously radiused from the interface 40 of theradiused segment 26 with a first straight segment 22 to the interface 42of the radiused segment 26 with a second straight segment 24. In thisform, the cycle 20 is pseudo-sinusoidal relative to the axis 32, withboth a positive portion (positive y-direction with respect to the axis32 at the centerline) and a negative portion (negative y-direction withrespect to the axis 32 at the centerline). FIG. 2 is a plan, schematicview of another exemplary embodiment of a heating element 10. The cycles20 in the exemplary embodiment shown in FIG. 2 have radiused segments 26that include both straight portions and continuously radiused portionsfrom the interface 40 of the radiused segment 26 with a first straightsegment 22 to the interface 40 of the radiused segment 26 with a secondstraight segment 24. In this form, the cycle 20 is pseudo-squarerelative to the axis 32, with the change in path direction in the lobes34 approaching a squared-off geometry, with both a positive portion anda negative portion.

The radiused segments 26 in both the pseudo-sinusoidal and thepseudo-square form of the heating element 10 have both an interiorradius r₁ and an exterior radius R₂. Each radiused segment 26 has anassociated angle α that represents the angular change in direction ofthe circuitous path over the length L_(r) of the radiused segment 26.With regard to individual radiused segments 24, exemplary embodiments ofthe radiused segments have an angle α that is between 90 degrees and 135degrees, i.e., 90°<α<135°, alternatively between 90 degrees and 100degrees, i.e., 90°<α<100°,

In exemplary embodiments, an angular sum β of the angles α of one lobe38 is greater than 180 degrees, preferably greater than 180 degrees toabout 200 degrees, more preferably about 185 to about 190 degrees. Forexample, the angular sump of a lobe 38 can be expressed as:

β=Σα_(n)

where n=number of radiused segments in the lobe. As each cycle includestwo lobes, an angular sum of the angles α associated with a single cycleof the circuitous path is greater than 360 degrees, preferably greaterthan 360 degrees to about 400 degrees, more preferably about 370 toabout 380 degrees.

The angular sum β greater than 180 degrees results in the two firststraight segments 22 adjacent the lobe 38 being non-parallel. Thisnon-parallel relationship can be seen in both FIGS. 1 and 2. Towards theinner surface of the lobe 38, the two first straight segments 22adjacent the lobe 38 are separated by a distance D₁ that is greater thana distance D₂ separating the same two first straight segments 22 nearthe mouth 44 of the opening 46 between lobes 38 in consecutive positiveor negative portions. D₁ is measured at one end of the first straightsegments 22 and D₂ is measured at a second end of the first straightsegments 22.

The circuitous path of the heating element 10 can be idealized as a line50 located at a centerline of the planar heating element 10. FIGS. 1 and2 illustrate the location of the line 50 in the illustrated embodiments.This line 50 can be used to measure the distance of the circuitous pathas well as to measure the angles α of the radiused segments 26 and thelengths L₁ of first straight segments 22, the lengths L₂ of the secondstraight segments 24 and the lengths L_(r) of the radiused segments 26.A length L₁ of the first straight segment 22 is greater than a length L₂of the second straight segment 24.

It can be understood by one skilled in the arts that the uniformity ofpower dissipation of an emitter is higher for a homogenous conductor ofuniform cross-section and surface area. It is therefore desirable tomaximize the ratio of the length of straight segments to the length ofcurved segments. It has been determined empirically that the followingrelationship yields a result with high uniformity, high fill-factor(ratio of substrate surface power to emitter surface power) andminimizes stress in the emitter. Furthermore, this relationshipaccommodates and controls expansion during transient conditions and overthe useful life of the heating element.

In exemplary embodiments, the lengths of the first straight segments 22,the second straight segments 24 and radiused segments 26 in a singlecycle are such that they satisfy the following relationship:

$\frac{\left( {L_{1,A} + L_{1,B} + L_{2,4} + L_{2,B}} \right)}{\left( {L_{r,a} + L_{r,b}} \right)} > 2.0$

where L_(1.A) is the length L₁ of a first first straight segment 22,L_(1.B) is the length L₁ of a second first straight segment 22, L_(2,A)is the length L₂ of a first second straight segment 24, L_(2,B) is thelength L₂ of a second second straight segment 24, L_(r,a) is the lengthL_(r) of a first radiused segment 26 and L_(r,b) is the length L_(r) ofa second radiused segment 26. Alternatively, the relationship above isgreater than 2.2, further from greater than 2.2. to less than 10.0 orless than 5.0. This relationship represents the ratio of the length ofstraight segments to the length of radiused segments. For a uniformwidth of the emitter surface, this is also the ratio of surface areas ofstraight segments to radiused segments. An example of a suitable widthis 8 mm. The length is measured at the center of the emitter path, i.e.,along line 50.

FIGS. 3A-B and 4A-B schematically illustrate from two differentperspectives, disassembled views of two exemplary embodiments of aheating assembly. The heating assembly 100 includes a heating element 10and insulation 102. The insulation 102 can be any suitable insulation.In an exemplary embodiment, the insulation 102 includes an insulatingsubstrate 104 with an alumina surface layer 106. Other suitableinsulation 102 includes a substrate formed from an insulating material,preferably a ceramic fiber composite, with an alumina facing layer and ablended ceramic fiber backing layer. The heating element 10 can be anysuitable heating element substantially consistent with the heatingelement 10 disclosed and described herein, e.g., as in any one of FIGS.1 and 2. As shown in FIGS. 3A-B, 4A-B and 8 the heating element ispseudo-sinusoidal as in FIG. 1.

The heating element 10 includes a plurality of mounting members 14. Themounting members 14 extend from the periphery 60 of the continuous strip12 at a plurality of locations along the circuitous path. The mountingmembers can be located at any suitable position. In one embodiment andas shown in FIGS. 1-4, the mounting members 14 are on an inner edge ofthe path proximate a maximum lateral position of the path, i.e., at ornear the maximum position of positive and/or negative portions of thelobes 38. Alternatively, the mounting members 14 are on an outer edge ofthe path proximate a maximum lateral position of the path, i.e., at ornear the maximum position of positive and/or negative portions of thelobes 38. In a further alternative, the mounting members 14 are onalternating peripheral edges of the continuous strip, preferablyproximate a maximum lateral position of the path, i.e., at or near themaximum position of positive and/or negative portions of the lobes 38.

As shown in FIGS. 3A-B and FIGS. 5A-C, the plurality of mounting members14 can be integrally formed with the continuous strip 12 and extend fromthe periphery 60 of the continuous strip 12 in substantially a secondplane.

In a first embodiment, the mounting members 14 include a base end 62 anda distal end 64 and have an integrated spacer 66 at the base end 62. Alength the integrated spacer 66 extends from the base end 62 defines astand-off distance for the continuous strip 12 when the heating element10 is mounted to a substrate 102. A washer 68 or other plane surface canbe optionally included to prevent the integrated spacer 66 fromembedding into the insulation 102. FIG. 5A shows the integrally formedmounting member 14 extending approximately 90 degrees out of plane fromthe continuous strip 12. In FIG. 5B, a washer 68 has been placed on themounting member 14, which functions to prevent the integrated spacer 66from embedding into the insulation 102. FIG. 5C shows a portion of afully assembled heating assembly 100 with the distal end 64 penetratingthe insulation 102 until the spacer 66 and the washer 68 contact asurface of the insulation 102 to form the stand-off distance D_(O).Integrated mounting means are advantageously installed in shapes thatare substantially flat with respect to the lateral dimension of thepattern.

As shown in FIGS. 4A-B and FIG. 6, the plurality of mounting members 14can be separate elements, including an opening 70 integrally formed onthe continuous strip 12 and an extension assembly including a pin 72 anda spacer 74. To mount the continuous strip 12 to the insulation 102, thepin 72 is operably positioned in the opening 70 to extend insubstantially a second plane. The pin 72 also anchors a spacer 74 inposition, which provides a stand-off distance between the heatingelement 10 and the insulation 102. Another washer 76 can optionally beon an opposite side of the insulation 102 to provide support and/oranchoring. When the mounting members 14 are separate elements, themounting members 14 can incorporate ceramic, metallic and/or compositestructures. Another example of a suitable spacer is a length of tubingequal to the desired stand-off distance between the heating element andthe substrate. Mounting members with separate elements areadvantageously installed in shapes that are substantially curved withrespect to the lateral dimension of the pattern.

As used herein, the second plane is different from the first plane inwhich the emitting surface 30 generally extends and generally iscontained. As an example, the first plane is oriented substantiallyconsistent with an XY-plane and the second plane is substantiallyconsistent with a YZ-plane or a XZ-plane of a right-handed,three-dimensional Cartesian coordinate system.

In a further exemplary embodiment, a combination of the integratedmounting members, such as mounting members 14 shown in FIG. 1, andmounting members that are separate elements, such as mounting members 14shown in FIGS. 4A-B, can be used on the same heating element 10. In sucha case, the integrated mounting members could be used to hold theheating element in place, while the mounting members that are separateelements could be added once the heating element is installed to theinsulation. A combination of integrated mounting members and mountingmembers that are separate elements could be advantageous for non-flatinstallations.

FIGS. 7A-D show, in plan view, several embodiments of a heating element10 with a combination of types of integrated mounting members andmounting members that are separate elements. FIGS. 7A-D illustrate themounting members prior to any bending of such members for installation.FIGS. 7A-D show variations in the location of the integrated mountingmembers. Thus, in FIG. 7A, the integrated mounting members 14 projectsfrom an outer peripheral edge of the lobe; in FIG. 7B, the integratedmounting members 14 projects from an outer peripheral edge of the firststraight segment; in FIG. 7C, the integrated mounting members 14projects from an outer peripheral edge of the second straight segment;and in FIG. 7D, the integrated mounting members 14 projects from aninner peripheral edge of the second straight segment. FIGS. 7A-D alsoshow variations in the orientation of the integrated mounting members.Thus, in FIGS. 7A and 7B, the integrated mounting members 14 areoriented substantially parallel with the axis 32 of the heating elementand in FIGS. 7C and 7D, the integrated mounting members 14 are orientedsubstantially perpendicular to the axis 32 of the heating element.

FIG. 8 shows a perspective, disassembled view of another exemplaryembodiment of a heating assembly 100 with a combination of types ofintegrated mounting members and mounting members that are separateelements, such as, for example, shown and described in one of theheating elements 10 in FIGS. 7A-D.

The heating element 10 comprises a power terminal 110 at the first end34 or the second end 36 of the heating element 10. In alternativeembodiments, the power terminals are located at locations other than thefirst end or second end. The power terminal connects to an electricalcircuit of, for example, a semiconductor processing furnace. FIGS. 3, 4and 6, for example, show examples of a suitable power terminal 110. Theheating element is heated by inducing an electrical current through it.In the disclosed embodiment, the electrical current is induced from adirect-coupled ac power source, but other methods and power sourcescould be employed such as direct coupled DC and inductively coupledpower sources.

The heating element 10 can be formed from any suitable material. Forexample, in an exemplary embodiment, the heating element 10 is formedfrom a resistance alloy, preferably an iron chromium aluminum alloy.Other suitable resistance alloys include nickel chromium or ceramicalloys, such as molybdenum disilicide or silicon carbide. The resistancealloy can be formed into a heating element body by, for example, cuttingthe heating element body from a sheet of material, casting a heatingelement body, machining a heating element body, extruding, pressing,punching or canning a heating element body, or combinations of suchmethods.

Embodiments of the disclosed heating element and heating assemblyprovide several advantages, either singly or in combination. Forexample, the pseudo-sinusoidal and pseudo-square patterns comprise asubstantial portion of straight segments with substantially uniformwidth yielding highly uniform surface temperatures. FIGS. 9A-B and 10show the temperature profile for both pseudo-sinusoidal patterns (FIGS.9A and 9B) and pseudo-square patterns (FIG. 10). Note the differentmounting members between the embodiment of FIG. 9A and the embodiment ofFIG. 9B. For comparison, a temperature profile for a conventional designconsisting primarily of curved segments is shown in FIG. 11. All of thetemperature profiles are from a temperature profile model using thefollowing parameters: current of 100 A and a furnace temperature of1000° C.

In the pseudo-sinusoidal pattern, the difference between the highest andthe lowest temperature (ΔT) along the emitting surface is about 8° C.(FIG. 9A) and about 7° C. (FIG. 9B). The temperature variation can berepresented as ΔT/T_(max) and is 0.79% for FIG. 9A and 0.69% for FIG.9B. Values for the same parameters for the pseudo-square pattern in FIG.10 are ΔT of about 33° C. and ΔT/T_(max) of 3.09%. In contrast, thevalues for these parameters for a conventional design consistingprimarily of curved segments (FIG. 11) are ΔT of about 21° C. andΔT/T_(max) of 2%. As seen from these figures, the temperature is mostuniform for the pseudo-sinusoidal pattern (FIGS. 9A-B). While thetemperature uniformity of the pseudo-square pattern (FIG. 10) is lessuniform than for the conventional design consisting primarily of curvedsegments (FIG. 11), it still has an advantage over the conventionaldesign because it better accommodates thermal expansion.

Heating elements disclosed herein can be incorporated into a furnace,such as a furnace for processing semiconductors. In such an application,multiple heating elements are positioned in an array or zone and arecontrolled for heating by a temperature control circuit. FIGS. 12A and12B show two different perspective views of an exemplary embodiment of aheating assembly with a plurality of heating elements arranged in anarray or zone.

FIGS. 13A-E show examples heating element installations. FIG. 13A showsan exemplary cylindrical installation 200 with staggered heatingelements 10 arranged in a circumferential direction, i.e., the axis ofthe heating element is not parallel to the axis of the cylinder. Thestaggered heating elements 10 are more clearly seen where the ends ofthe heating elements are visible, such as at location 202. Staggeringcontributes to minimizing non-uniformity caused by the void in emittersurface at the terminal end of the heating element by distributing thisterminal end position within the assembly across the heating surface.The size of the stagger can be smaller or greater, depending on theapplication and the desired result. Further, a single heating elementcould be used to go around the circumference one or more times asopposed to using two or more semicircular segments. FIG. 13B showsanother exemplary cylindrical installation 204 with non-staggeredhearing elements 10 arranged in a circumferential direction, i.e., theaxis of the heating element is not parallel to the axis of the cylinder.FIG. 13C shows an exemplary semi-cylindrical installation 206 withheating elements 10 arranged axially, i.e., the axis of the heatingelement is parallel to the axis of the cylinder. FIG. 13D shows anexemplary semi-cylindrical installation 208 with heating elements 10arranged in a circumferential direction, i.e., the axis of the heatingelement is not parallel to the axis of the cylinder. FIG. 13E shows anexemplary planar-angled installation 210 with heating elements 10 onadjoining planar sections 212 a, 212 b, 212 c being oriented indifferent directions. For example, the axes of the heating elements 10in a first planar section 212 a can be non-parallel, alternativelyperpendicular, to the axes of the heating elements 10 in a second planarsection 212 b. The planar-angled installation 210 one can, for example,be used to approximate cylinder and semi-cylinders installations. Theseinstallations are merely illustrative and any installation arrangementcan be used that obtains the desired heating and temperature profile.

The radii of the radiused segments are sized to maximize temperatureuniformity and minimize stress. The inside radii have a particular lowstress when compared to the conventional designs consisting primarily ofcurved sections. Further, the uniformity of surface temperatures is muchimproved relative to conventional designs consisting of square patternswith little or no radii at the corners.

The heating elements disclosed herein have a high surface loadingfactor, also known as a fill factor. Here, the pseudo-square andpseudo-sinusoidal heating elements have more emitter surface area thanconventional designs consisting primarily of curved sections. This is,at least in part, because the angular sum of a single cycle is greaterthan 360 degrees. This puts the first straight segments in non-parallelrelationship with a resulting longer length than parallel segments, andtherefore, more emitting surface. Further, the distance D₂ is minimizedwhile the distance D₁ is varied to accommodate the length (L₂) of thefirst straight segment 22. This contributes to a high fill factor whileincreasing temperature uniformity and lowering stress in the lobes 38.An example of a typical surface loading of total active area isapproximately 145% of the emitter loading.

The heating elements disclosed herein contribute to controlling thermalexpansion effects. Materials forming the heating element expand uponheating proportional to the coefficient of thermal expansion of thematerial. This expansion can cause the heating element to flex and bend,resulting in the emitter surface having a variable position relative toa piece to be heated (and, therefore, making the temperature profilemore non-uniform). In extreme situations, the flexing and bending canresult in short circuiting. The heating elements disclosed hereincontrol and minimize the effects the thermal expansion. For example, thenon-parallel orientation of the first straight segments direct a portionof the thermal expansion in the lateral direction into the longitudinaldirection, and therefore maintains the orientation relative to the pieceto be heated and a more uniform temperature profile. In another example,the edges of the pattern can be curved or bent along the lateral axis inorder to direct the thermal expansion toward the insulating substrate topermit placement of the heating element closer to adjacent objects, suchas additional circuits, with reduced instance of short circuiting.

The use of mounting means with a stand-off distance can also contributeto improved performance. Alternating support locations along the lengthof the circuitous path allows thermal expansion of the heating elementto be directed into a twisting or torsional movement of the heatingelement between the supports and not just in planar movement.

The heating elements disclosed herein are free-radiating. That is, themounting members provide a stand-off distance for the heating elementrelative to the insulating substrate. The architecture allows for heatto emit from all sides evenly and without the use of extra electricalenergy to compensate for, for example, heating a substrate in surfacecontact to the heating element. Thus, the free-radiating heating elementlowers the operating temperature of the emitter. Further, such a heatingelement will have a longer life at the same substrate power density asconventional heating elements or, alternatively, can operate at higherpower densities over comparably life times.

The disclosed embodiments result in a high performance heating elementcombining low mass and high surface area. The disclosed patterns enablea high degree of automation in the fabrication and assembly process andprecise geometries yielding uniform heating and consistent performance.

Several variations of the heating element can be made. For example, theheating element can have different thicknesses, varying, for example,from about 0.5 mm to about 10 mm. Also for example, the heating elementcan have different widths, based on the width of the straight segments,varying, for example, from about 5 mm or longer. These variations inwidth and thickness can be suitably incorporated as long as the basicfeatures of the geometry are maintained, i.e., the non-parallel firststraight segments and a substantially straight overall pattern.

While the drawing figures disclose embodiments that are substantiallyplanar in configuration, it will be appreciated by those skilled in thatart that the disclosed geometries can be applied to assemblies that havecurved surfaces such as cylinders or semi-cylinders. The variation thatincorporates the separate mounting means specifically lends itself tothose configurations by allowing the emitter to be appropriately formedto conform to the curved surface, and then fixed in place by theseparate mounting means. Curved surface configurations may also beconstructed by arranging the emitter segments so that they run along theaxial length of the curved surface, or by approximating the desiredcurved geometry with a series of planar panels.

Although described in connection with preferred embodiments thereof, itwill be appreciated by those skilled in the art that additions,deletions, modifications, and substitutions not specifically describedmay be made without department from the spirit and scope of theinvention as defined in the appended claims.

1. A heating element, comprising: a continuous planar strip, wherein apath of the continuous strip from a first end to a second end iscircuitous and includes a plurality of repeating cycles, each repeatingcycle including a plurality of first straight segments, a plurality ofsecond straight segments and a plurality of radiused segments, wherein alength of the first straight segment is greater than a length of thesecond straight segment, and wherein an angular sum of a single cycle ofthe circuitous path is greater than 360 degrees.
 2. The heating elementof claim 1, wherein the plurality of first straight segments include afirst group that is non-parallel to a second group.
 3. The heatingelement of claim 1, wherein the plurality of first straight segments areoriented generally laterally to an axis oriented from a first end of theheating element to a second end of the heating element and wherein theplurality of second straight segments are oriented generallylongitudinally to the axis.
 4. The heating element of claim 1, whereinthe plurality of second straight segments are oriented generallylongitudinally to an axis orienting from a first end of the heatingelement to a second end of the heating element and any two consecutivesecond straight segments are parallel.
 5. The heating element of claim1, wherein two radiused segments and one second straight segment form alobe, wherein the single cycle has two lobes and wherein the one secondstraight segment separates the two radiused segments.
 6. The heatingelement of claim 5, wherein an angular sum of one lobe is greater than180 degrees, preferably greater than 180 degrees to about 200 degrees,more preferably about 185 to about 190 degrees.
 7. The heating elementof claim 6, wherein an angular sum of one lobe is greater than 180degrees to about 200 degrees.
 8. The heating element of claim 1, whereinan angle of the radiused segments is between 90 degrees and 135 degrees.9. The heating element of claim 1, wherein a ratio of a sum of thelengths of the two first straight segments and one second straightsegment in a single cycle to a sum of the lengths of two radiusedsegments is greater than 2.0.
 10. The heating element of claim 9,wherein the ratio is greater than 2.2.
 11. The heating element of claim1, comprising a plurality of mounting members, wherein the plurality ofmounting members extend from a periphery of the continuous strip at aplurality of locations along the circuitous path.
 12. The heatingelement of claim 11, wherein the circuitous path is sinuous-like and theplurality of locations are on an inner edge of the sinuous-like pathproximate a maximum lateral position of the sinuous-like path.
 13. Theheating element of claim 11, wherein the circuitous path is sinuous andthe plurality of locations are on an outer edge of the sinuous pathproximate a maximum lateral position of the sinuous-like path.
 14. Theheating element of claim 11, wherein successive locations of theplurality of mounting means are on alternating peripheral edges of thecontinuous strip.
 15. The heating element of claim 11, wherein anemitting surface of the continuous planar strip extends in and iscontained in a first plane, and wherein the plurality of mountingmembers are integrally formed with the continuous strip and extend fromthe periphery of the continuous strip in substantially a second plane,the second plane different from the first plane.
 16. The heating elementof claim 15, wherein the mounting members include a base end and adistal end and includes an integrated spacer at the base end.
 17. Theheating element of claim 16, wherein a length the integrated spacerextends from the base end defines a stand-off distance for the heatingelement when the heating element is mounted to a substrate.
 18. Theheating element of claim 11, wherein an emitting surface of thecontinuous planar strip extends in and is contained in a first plane,and wherein the plurality of mounting members include an openingintegrally formed on the continuous strip and an extension assemblyincluding a pin and a spacer, wherein the pin is operably positioned inthe opening to extend in substantially a second plane, the second planedifferent from the first plane.
 19. The heating element of claim 3,wherein the circuitous path is pseudo-sinusoidal relative to the axis.20. The heating element of claim 3, wherein the circuitous path ispseudo-square relative to the axis.
 21. The heating element of claim 1,wherein the heating element is formed from a resistance alloy,preferably an iron chromium aluminum alloy.
 22. The heating element ofclaim 1, comprising a power terminal at the first end or the second endof the heating element.
 23. A heating assembly comprising: an insulatingsubstrate; and a heating element, including a continuous planar strip,wherein a path of the continuous strip from a first end to a second endis circuitous and includes a plurality of repeating cycles, eachrepeating cycle including a plurality of first straight segments, aplurality of second straight segments and a plurality of radiusedsegments, wherein a length of the first straight segment is greater thana length of the second straight segment, and wherein an angular sum of asingle cycle of the circuitous path is greater than 360 degrees, whereinthe heating element is mounted in spaced relation to the insulatingsubstrate by the plurality of mounting members.
 24. The heating assemblyof claim 23, wherein the insulating substrate is formed from aninsulating material, preferably a ceramic fiber composite with analumina facing layer and a blended ceramic fiber backing layer.
 25. Theheating assembly of claim 23, wherein the plurality of first straightsegments include a first group that is non-parallel to a second group.26. A method of manufacturing a heating assembly, the method comprising:forming a heating element body from a resistance alloy, the heatingelement body including a continuous planar strip with an emittingsurface and a plurality of mounting members; bending the plurality ofmounting members out of plane relative to the continuous strip; andinserting the plurality of mounting members into a substrate until anintegrated spacer on the mounting members contacts the substrate,wherein a path of the continuous strip from a first end to a second endis circuitous and includes a plurality of repeating cycles, eachrepeating cycle including a plurality of non-parallel first straightsegments, a plurality of second straight segments and a plurality ofradiused segments, wherein a length of the first straight segment isgreater than a length of the second straight segment, and wherein anangular sum of a single cycle of the circuitous path is greater than 360degrees.
 27. The method of claim 26, further comprising attaching apower source to the heating element.
 28. A method of manufacturing aheating assembly, the method comprising: forming a heating element bodyfrom a resistance alloy, the heating element body including a continuousplanar strip with an emitting surface; and inserting a plurality ofmounting members through an opening integrally formed on the continuousstrip and into a substrate until a spacer associated with the mountingmembers contacts the substrate, wherein a path of the continuous stripfrom a first end to a second end is circuitous and includes a pluralityof repeating cycles, each repeating cycle including a plurality ofnon-parallel first straight segments, a plurality of second straightsegments and a plurality of radiused segments, wherein a length of thefirst straight segment is greater than a length of the second straightsegment, and wherein an angular sum of a single cycle of the circuitouspath is greater than 360 degrees.
 29. The method of claim 28, furthercomprising attaching a power source to the heating element.